Method for forming a plurality of electrostatic latent images on an electrophotographic plate

ABSTRACT

A plurality of electrostatic latent images are formed on the surface of an electrophotographic plate having an electrically insulating overcoating by a method comprising the steps of charging the plate to a uniform surface potential, exposing to a first image pattern, recharging the plate to restore the uniform surface potential and subsequently exposing to a second image pattern. The latent images can be developed with an electroscopic marking material to form visible powder images which can be fixed to the plate or transferred to a recording medium and fixed thereon.

United States Patent [191 Gundlach et al.

[ METHOD FOR FORMING A PLURALITY OF ELECTROSTATIC LATENT IMAGES ON ANELECTROPHOTOGRAPHIC PLATE [75] Inventors: Robert W. Gundlach, Victor;Lloyd F. Bean, Rochester, both of NY.

[73] Assignee: Xerox Corporation, Stamford,

Conn.

22 Filed: Aug. 25, 1971 [21] App1.No.: 174,830

Related US. Application Data [63] Continuation-impart of Ser. No.50,216, June 26,

1970, abandoned.

[52] US. Cl 96/l.4, 96/1 R, 117/175 [51] Int. Cl G03g 13/22 [58] Fieldof Search 96/1, 1.5, 1.4; 117/37,

[56] References Cited UNITED STATES PATENTS 2,624,652 1/1953 Carlson96/1 R June 11, 1974 3,121,010 2/1964 Johnson et al. 96/1 R 3,438,7064/1969 Tanaka et al. 96/! X 3.457.070 7/1969 Watanabe et al 96/l.5

Primary Examiner-Charles E. Van Horn Attorney, Agent, or Firm-James J.Ralabate; David C. Petre; Gaetano D. Maccarone [57] ABSTRACT to arecording medium and fixed thereon.

17 Claims, 8 Drawing Figures PATENTEflJuu 1 1 m4 SHEET 10F 2 FIG.

INVENTORS. ROBERT W. GUNDLACH LLOYD F. BEAN 7AQCMM ATTORNEY mm .mm m

SHEET 2 OF 2 ZERO T FIG. 3

ZERO v Pl m J 6,24

FIG. 4

ZERO

P2 I H/ i V- cl METHOD FOR FORMING A PLURALITY OF ELECTROSTATIC LATENTIMAGES ON AN ELECTROPHOTOGRAPI-IIC PLATE BACKGROUND OF THE INVENTIONThis application is a continuation-in-part of earlier copendingapplication Ser. No. 50,216, filed June 26, 1970 and now abandoned.

This invention relates in general to electrostatic images such as areuseful in xerographic reproduction and, in particular, to a method forforming a plurality of electrostatic latent images on the surface of axerographic plate.

In the practice of xerography it is the general procedure to form anelectrostatic latent image on a xerographic plate surface generally bycharging a photoconveloped with an electroscopic marking material toform a visible powder image thereon which is subsequently transferred toa recording medium and fixed thereto, the cycle being repeated anynumber of times to produce any desired number of reproductions of theoriginal image. New developments in information reproduction have nowmade it advantageous to copy two original images on one recording mediumsuch as where computer-generated information in intended to be copied inconjunction with special forms. This has been accomplished heretofore byusing overlay transparencies corresponding to the form together with theoriginal information sought to be inserted thereon and exposing acharged xerographic plate to the combination. Highly satisfactoryresults are obtained according to this procedure; however, thetransparency must initially be made prior to reproduction of theinformation on the forms thus involving delay and aided expense. Thereare. also problems with respectto maintaining intimate contact oftheoverlay with the opaque reproduction subject during exposurewhich canlead to shadow effects, i.e., double lines in the finished copy. Itwould be desirable to have a method for copying original images incombination whereby electrostatic latent images could be formed on axerographic plate directly from the originalimages desired to bereproduced.

SUMMARY OF THE INVENTION Now according to the present invention it ispossible to form, on a single xerographic plate, electrostatic latentimages corresponding to a plurality of original images. The originalimages may be positives or it is likely in many instances such as thesituation discussed above that at least one image can be as readily madeavailable as a negative rather than as a positive. It is therefore. anobject of the invention to provide a novel method for formingelectrostatic images whereby the abovementioned needs are fulfilled andthe disadvantages of the prior known methods are overcome.

It is another object of the invention to provide a method for forming,on the surface of an overcoated xerographic plate, a plurality ofelectrostatic latent images corresponding to a plurality of originalimages.

A further object of the invention is to provide such an imaging methodwherein development of the visible images may be carried out by anydevelopment system.

Still another objectof the invention is to provide such an imagingmethod wherein development of the visible images is accomplished by adevelopment system which responds to absolute differences from areference potential.

Yet a further object is to provide a xerographic reproduction methodwherein a plurality of images can be reproduced concurrently on arecording medium.

The foregoing objects and advantages are realized according to theinvention by employing an overcoated xerographic plate comprised of aphotoconductive insulating layer on a conductive substratewith a film ofelectrically insulating material applied over the photoconductive layerand forming on the plate a plurality of electrostatic latent imagescorresponding to a plurality of original images. Briefly stated, thexerographic plate is uniformly electrostatically charged in the absenceof illumination, exposed to activating radiation in a patterncorresponding to a first original image, charged a second time in theabsence of illumination to restore the plate surface to a uniformpotential and subsequently exposed to activating radiation in a patterncorresponding to a second original image whereby the electrostaticlatentimages are recorded on the plate as will become clearly apparent.

According'to a preferred embodiment of the inven- I tion the first andsecond original image patterns may be both positive in image sensewhereas in another preferred embodiment the first image pattern may benegative in image sense and the second pattern a positive.

der images which can be fixedto the plate or which can be transferred toa recording medium and fixed thereon, thus allowing the plate tobereused.

Other objects and advantages of the invention will become readilyapparent from the following detailed description of the preferredembodiments thereof when read with reference to the drawings in which:

FIG. 1 is a side view of a xerographic plate employed .in the practiceof the invention;

dition of various areas of the plate upon completion of FIG. 4 is agraphical illustration representing the condition of various areas ofthe plate upon completion of the practice of still another embodiment ofthe invention; and

FIG. 5 is a graphical illustration representing the condition of variousareas of the plate upon completion of the practice of yet anotherembodiment of the invention.

The xerographic plates which are suitable for use according to theinvention are constructed generally as illustrated in FIG. 1 andcomprise a photoconductive insulating layer on a conductive backing 12with a highly electrically insulating film 14 coated over thephotoconductive layer. The film 14 may be a coating of the naturedisclosed in U.S.Pat. No. 2,860,048 to Deubner. The coating maybeformedby any bonding method thatwill eliminate space between the film and thelayer without significantly altering the electrical or photoelectricalcharacteristics of either. In one embodiment of the plate theovercoating is transparent to the activating radiation thus permittingit to reach the photoconductive layer when the plate is illuminated fromthe film side; or in another embodiment the plate can have a substrate12 which is transparent to the activating radiation allowing the plateto be illuminated from the substrate side as is known in the art.Insulating film 14 can be any of many coating materials such a polyvinylchloride, polystyrene, polytetrafluoroethylene, polyethyleneterephthalate or like resins or plastic materials and may be opaque ortransparent. When the film is opaque exposure of the photoconductivelayer may be effected through a transparent substrate, etc. The filmmaterial is preferably resistant to 4 ate shape in which case it can besecured to a rotating cylinder as is well-known in the art.

In a particularly preferred embodiment of the invention it is utilizedto combine computer-generated inforcleaning and abrasion and nonreactivewith the electroscopic marking material employed to develop the latentelectrostatic images whereby the plate can. be utilized in a recycliblemethod for xerographic reproduction to form a multiplicity ofreproductions. Photoconductive insulating layer 10 maybe constructedfrom any standard photoconductive materials such as, for

example, vitreous selenium, sulfur, anthracene and tel-- lurium; or itcan be a finely ground photoconductive insulating material dispersed ina high resistance electrical binder such as are disclosed in U.S. Pat.No. 3,121,006 to Middleton et al., or an inorganic photoconductivepigment dispersed in a photoconductive insulating materialsuch as aredisclosed in U.S. Pat. No. 3,121,007 to Middleton et al., or an organicphotoconductor such as phthalocyanine in a binder; or generally anyphotoconductive insulating material which is suitable for use inxerographic reproduction techniques.

The material selected for use as photoconductive insulating layer 10 inany instance will be, to some extent, determined by the particularembodiment of the invention being practiced as .will become apparentfurther below. The overcoated xerographic plates which can be employedin the method of the invention may have relative electrical thicknessesof the insulating film and the photoconductive layer of from about 4:1to about 1:4 with the preferred range being from about 2:1 to about 1:2and the optimum embodiment consisting of a plate which has about a 1:1relationship. The xerographic plate, which may be rigid or flexible, maybe flat or it can have other configurations such as an arcumation onstandard forms. In such a situation the computer-generated informationtypically would be negative in image sense. Additionally, it isparticularly preferred that the initial and subsequent charging steps beof the same polarity because materials considerations, e.g., theparticular photoconductive insulating materials used are simplified andmore latitude is provided with respect to the type of developmentsystems which can be employed. Therefore, for purposes of illustration,the invention will be described in detail with respect to the embodimentwherein the first original image pattern is negative in image sense, thesecond original image pattern is positive in image sense and bothcharging steps apply a potential of the same polarity.

The invention will be more easily understood when described in relationto FIG. 2A through FIG. 2D wherein the charge density 0' and thepotential y at any position x on the free surface of the plate are shownby dotted and solid lines respectively. Referring now to FIG. 2A it isseen that the charge density and potential across the surface of anovercoated xerographic plate of the type previously described areuniform after the plate has been electrically charged in the absence ofil- Iurnination by a conventional method such as that disclosed in U.S.Pat. No. 2,777,957 to Walkup but which may be any other plate chargingmethod. The plate may be charged positively or negatively with thecharge density placed on the plate preferably being the charge giving aninternalfield of about 25 volts/micron, but which may vary from asurface charge giving internal fields of about 5 to 10 volts/micron'toan upper limit of about volts/micron or the maximum electric field thatthe given dielectrics will effectively support without breakdown. Theuniformly charged plate is then exposed to a pattern of. activatingradiation corresponding to at least one original negative image, thatis, bright characters on a dark background, the source of illuminationbeing any to which the photoconductive insulating layer 10 is sensitive.As shown by FIG. 2B the charge density remains uniform across the plateas will be understood by those skilled in the art whereas the areas ofthe photoconductor which are exposed to the activating radiation,represented by E are discharged causing the potential of the freesurface in these areas to drop. The non-exposed portions of the platesurface experience a negligible drop in potential. Thus, voltagegradients representative of the pattern of activating radiation incidentupon the plate are created thereby giving rise to the formation of anelectrostatic latent image. The condition of the area represented by Eis intended to be illustrative of the situation which exists at any areaof the plate where the activating illumination has reached thephotoconductive layer.

The negative image or images may typically be projected from a cathoderay tube (CRT) display or from a microfilm transparency. Where a numberof original negative images are'desired to be combined in the final copythey may be projected simultaneously or sequentially on the plate. Itshould be appreciated by those skilled in the art that any number oforiginal negative images may be projected onappropriate areas of theplate during this step of the method. For example, it is possible toproject certain information on various parts of the plate sequentiallyor simultaneously with the projection of a standard form thereon.

The xerographic plate is then charged again to a uniform potential inthe absence of illumination in the manner previously described.According to this preferred embodiment of the invention the polarity ofthe charge applied in this charging step is the same as that of thecharge applied in the initial charging step. FIG. 2C illustrates thecondition of the plate after the second charging operation has beeneffected. The potential across the plate surface is again made uniformbut the charge density hasnow become non-uniform in a patternrepresentative of the activating illumination which reached thephotoconductive layer during the previous exposure step. The areas ofthe plate represented by E that is, those which had been exposed to theactivating radiation, have a greater charge density than those portionswhich did not receive any illumination. Subsequently the plate isexposed to a second pattern of activating radiation, this correspondingto an original positive image, that is, dark characters on a whitebackground. The positive image is positioned in a manner such as to bein registration with the original negative image or images to which theplate had previously been exposed. FIG. 2D shows the condition of theplate at this point. There now exist two sets of electrostatic latentimages on the plate as represented by E and D the latter intended to beillustrative of the portions of the plate which did not receive anyillumination during the second exposure, namely the image areas of thepositive image, each of the electrostatic latent images having apositive potential with respect to the background areas. Electroscopicmarking material having a charge of polarity opposite to that of thecharge placed on the plate is brought into contact with the platesurface as by cascading the material across'the plate, although itshould be recognized that any other xerographic development methodincluding, for example, magnetic brush and liquid development methodsmay be employed, thus forming visible powder images which can be fixedto the plate or which can subsequently be transferred to a recordingmedium by any conventional transfer method and fixed thereto. While itis often desirable to develop the electrostatic latent images with tonermaterial the images may be used in a host of other ways as, for example,electrostatic scanning systems may be employed to read the latent imagesor the images may be transferred by TESI techniques to other materialsand stored.

Where the xerographic plate is to be reused to make additionalreproductions as in a recyclible xerographic' method any residual chargeremaining on the plate after the visible powder images have beentransferred to a recording medium must be removed therefrom prior toeach repetition of the cycle. Generally the residual charge can beremoved from the plate by ionizing the air above the insulating filmwhile the photoconductive layer is uniformly illuminated and grounded.For example, the charge removal could be accomplished by AC. coronadischarge in the presence of illumination from a light bulb orpreferably a conductive brush can be brought into contact with the platesurface in the presence of such illumination. This latter mode ispreferred because it also cleans any toner particles remaining. Theinsulating layer can also be made sensitive to other light, e.g.,ultraviolet light and the residual charge removed by general exposure tothe appropriate illumination.

According to another embodiment of the invention the firstand secondoriginal image patterns to which the overcoated xerographic plate isexposed are positive in image sense "and the initial and subsequentcharging steps apply a potential of the same polarity. The condition ofthe plate after the practice of the method according to this embodimentis illustrated in FIG. 3. Referring now to FIG. 3 it can be seen thatthere exist two sets of electrostatic latent images on the plate asrepresented by A and A, with A, being representative of areas of theplate which do not receive any illumination during the first exposureand A being illustrative of those portions of the plate which do notreceive any illumination during the second exposure. Of course, in bothexposures the areas of the plate which do not receive any illuminationare those corresponding to image areas of the positive images. It isclearly evident that the potential contrast, with respect to a referencepotential P, between the image and background areas for each respectivelatent image is opposite in sign with one being positive with respect tobackground and the other negative with respect to background. Thus, theelectrostatic latent images can be developed simultaneously by anydevelopment system which develops absolute differences from a referencepotential. Typical suitable development systems which exhibit thischaracteristic behavior include polar liquid ink development andconductive powder development such as is described in US. Pat. No.3,166,432 to Gundlach.

FIG. 4 illustrates the condition of the plate after the practice ofanother embodiment of the invention wherein the first exposure is tonegative image input and the second exposure is to positive image inputwith the initial and subsequent charging steps applying po tentials ofopposite polarities from each other. In this illustrative instance thefirst charging step is carried out with a positive potential while thesecond is made with a negative potential. Of courseit will be understoodthat the respective directions of field may be applied in any order.Referring now to FIG. 4 it is seen that two sets of electrostatic imagesare formed on the plate as represented by B, and B B is intended to beillustrative of the condition'of the plate at any area where activatingillumination reaches the. photoconductive layer during the firstexposure step, i.e., any area of the plate corresponding to image areasof the negative image. B is representative of the plate areas which donot receive any illumination during the second exposure step, i.e., theplate areas corresponding to image areas of the positive image. Again itis seen that the potential contrast, with respect to a referencepotential P between the image and background areas for each respectivelatent image is opposite in sign. As was described above these imagescan be developed simultaneously by a development system which developsabsolute differences from a reference potential.

FIG. 5 is descriptive of the condition of the plate after the practiceof another embodiment of the invention wherein both exposure steps arecarried out with positive images and the initial and subsequent chargingsteps apply potentials of opposite polarities from each other. Although,for purposes of illustration, the first charging step is carried outwith a positive potential and the second with a negative potential, thereverse order may be used. Referring now to FIG. it is seen that twosets of electrostatic images are formed on the plate as represented byC, and C with C being representative of areas of the plate which do notreceive any illumination during the first exposure and C beingillustrative of portions of the plate which do not receive illuminationduring the second exposure. Both sets of electrostatic images arenegative with respect to reference potential P It should be recognizedthat these latent images may be developed by any development system.

with respect to the embodiments of the invention wherein potentials ofopposite polarities are applied in the respective charging steps it willbe appreciated by those skilled in the art that the photoconductivematerial employed in the xerographic plate typically should be one whichacts as an insulator for both directions of field in the dark. Typicalsuitable photoconductive materials which are capable of holding bothpolarities of charge in the dark include, for example, selenium andorganic binder plates containing cadmium sulfoselenide such as aredescribed in copending application Ser. No. 94,072 filed Dec. 1, 1970.

Although the present method is intended to be practiced in a manner suchthat registration of the respective images projected on the xerographicplate will preclude any overlap in the reproduced images it should berecognized that overlap may occur without substantially affecting theresults obtained. with respect to the reproduction of line copy whichtypically comprises the bulk of the copy reproduced, substantial areasof the original material are comprised of background. For example, a US.patent, on pages completely filled with single space information has abackground area in the order of about 90 percent with about percent ofthe total imaging area of the plage taken up by character information.Thus, it will be clearly evident to those skilled in the art that anyoverlap, between, for example, character information and a standard formwould not present any significant problems in the practice of theinvention.

It should be noted that the above described method has proved to be veryeffective through experimentation and any inaccuracy in the theoreticaloperation thereof as described and illustrated isnot to be construed asbeing limiting of the invention.

The invention will now be further described in detail with respect tospecific preferred embodiments thereof it being understood that theseare illustrative only and not intended to limit the scope of theinvention to the specific materials disclosed. All parts and percentagesare by weight unless otherwise indicated.

. EXAMPLE I A xerographic plate is constructed by arranging a 50 micronthick layer of amorphous selenium on a 50 mil thick aluminum sheet byvacuum evaporation and coating the exposed selenium layer surface with a12.5 micron thick film of poly-N-vinyl carbazole by solvent coatingtechniques using toluene. The poly-N-vinyl carbazole film and theselenium layer have a 1:2 relationship in electrical thickness. Theplate is charged positively to a potential of 1,000 volts by A.C. coronacharging in the absence of illumination and then exposed to actiniclight from a CRT, in the range of 450-500 nm, in a pattern correspondingto an original negative image of a form on which information is to be 8reproduced thus discharging the illuminated or exposed areas of theplate surface to a potential of about 350 volts with negligibledischarge in the background or non-image areas. The plate is againcharged positively to a uniform surface potential of 1,000 volts by A.C.corona discharge in the absence of illumination. Subsequently the plateis exposed to a camera projection of an original positive opaque imageof informa tion which is to be reproduced on the form, illuminated by afluorescent lamp. The positive image is positioned 'in a manner such asto be in registration with the original negative image to which theplate has been previously exposed. Two latent electrostatic images areformed on the plate, that corresponding to the original negative imageand the other corresponding to the original positive image havingpotentials of approximately 222 volts and 667 volts above the backgroundareas respectively. The latent electrostatic images are then developedby cascading an electroscopic marking material having a negative chargeof polarity across the plate surface thereby resulting in the formationof visible powder images which are transferred to a paper sheet andfixed thereto thus providing a good quality reproduction of the originalimages with a clean background.

EXAMPLE I! The procedure described in Example I is employed with respectto a xerographic plate constructed with a 12.5 micron thick layer of 5percent mixture of metalfree phthalocyanine (Commercially availableunder the tradename Monolite Fast Blue from Holland Suco Co., Holland,Mich.) in vinyl acetate, vinyl chloride (commercially available underthe tradename VYNS from Union Carbide Co.) arranged on a 50 ml thickbacking sheet of aluminum and overcoated with a 38 micron thick film ofMylar. The Mylar is bonded to the photoconductive binder plate byheating to a temperature at which it is soft (about 250C) and rollingthe softened material under pressure over the plate. The Mylar film hasa 4:1 electrical thickness relationship with the phthalocyanine layer.

A negative microfilm transparency is first projected on the plateemploying incandescent tungsten illumination. The second exposure is toan impact-printed black on white subject using incandescentillumination. Two latent electrostatic images are formed on the plate,that corresponding to the original negative image and the othercorresponding to the original positive image having potentials of aboutvolts and 800 volts above' the background areas respectively. A goodquality reproduction with clean background is formed.

EXAMPLE llI The procedure followed in Example I is again followed withthe exception that two original negative images are projected on theplate during the first exposure step. Initially the plate is exposed toactinic light from a CRT display followed by exposure to a negativemicrofilm transparency. A copy of comparable quality as that made inExample I is obtained.

EXAMPLE IV The procedure of Example I is followed with respect to axerographic plate constructed by arranging a 30 micron thick layer of 30percent mixture of cadmium sulfoselenide (CdSSe) (commercially availableunder the tradename 1020 Red Pigment from General Color Company, Ft.Wayne, Ind.) in a glass binder (identified as Harshaw Commercial Frit,Sample N862 available from Harshaw Chemical Company, Cleveland, Ohio) ona 50 mil thick stainless steel backing plate and coating the exposedsurface of a CdSSe layer with a 30 micron thick film of polystyrene. Thepolystyrene film and CdSSe layer have about a 1:1 relationship inelectrical thickness. Negative corona charging is used in this example.

The illumination of the plate is carried out in the same mannerdescribed in Example 11. Two latent electrostatic images are formed onthe plate, that corre sponding to the original negative image and theother corresponding to the original positive image having potentials ofapproximately 250 volts negative and 500 volts negative above thebackground areas respectively. A goodquality reproduction with cleanbackground is obtained.

EXAMPLE V A xerographic plate is constructed by arranging a 50 micronthick layer of a photoconductive composition made up of about 5 partscadmium sulfoselenide, about 5 parts phthalocyanine and about 5 partsselenium dispersed in about 100 parts of poly-N-vinyl carbazolesensitized with 2,4,7-trinitro-9-fluoroenone on a 50 mil thick aluminumsheet. The exposed surface of the photoconductive layer is coated with a12.5 micron thick film of poly-N-vinyl carbazole. The poly-N-vinylcarbazole film and the photoconductive layer have a 2:1 relationship inelectrical thickness. The plate is charged positively to a potential of600 volts by corona charging in the absence of illumination and thenexposed to a camera projection of a first original positive opaque imagethus discharging the illuminated or background areas of the platesurface to a potential of about 200 volts with negligible discharge inthe image areas. The plate is then charged negatively to a uniformsurface potential of 600 volts by corona discharge in the absence ofillumination. Subsequently the plate is exposed to a camera projectionof a second original positive opaque image illuminated by a fluorescentlamp. Two electrostatic latent images of negative polarity are formed onthe plate, that corresponding to the first original positive image andthe other corresponding to the second original positive image havingcontrast potentials of approximately 133.3 and 466.7 volts below thebackground areas respectively. Both electrostatic images are negativewith respect to the background potential.

EXAMPLE Vl The procedure described in Example V is again used with theexception that the first exposure of the plate is made to activatinglight from a CRT, in the range of 450-500-nm, in a pattern correspondingto an original negative image ofa form on which information is to bereproduced. Thus after the first exposure step the ex- V10 groundpotential. In otherwords' the former is-positive with respect to thebackground potential and the latter is negative withrespect to thebackground potential. The electrostatic images arethen developed by apolar liquid ink development system.

EXAMPLE VII The procedure described in Example V is again followed withthe exception thatin the second charging step the plate is chargedpositively to a potential of 600 volts. Two electrostatic images ofpositive polarity are formed, that corresponding to the first originalpositive image having a potential of about 133.3 volts below thebackground areas and that corresponding to the second original positiveimage having a potential of about 277.7 volts above the backgroundareas. In other words the former is negative with respectto thebackground potential whereas the latteris positivewith respect to thebackground potential. The electrostatic images are then developed by apolar liquid ink development systern.

Although the invention has been described with relation to variousspecific and preferred embodiments thereof, it is not intended to' belimited thereto but rather those skilled in the art will recognize thatvariations and modifications may be made therein which are within thespirit of the invention and-the scope of the appended claims. Forexample, while the invention has been described with relation to thecombination of information on certain forms it can be utilized toreproduce many other combinations of original images. In actualcommercial practice it is preferred to secure the overcoated xerographicplate to a rotating drum in accordance with well known techniqueswhereby it is possible to make multiplicity of reproductions by arecyclible xerographic reproduction method.

What is claimed is;

l. A method for forming a plurality of electrostatic latent images on axerographic member comprising the steps of a. applying a firstelectrostatic charge to a xerographic member in the absence ofillumination, said member comprising a conductive substrate and aphotoconductive insulating layer with an electrically insulating filmapplied over said layer;

images by depositing electroscopic marking particles thereon.

3. The method asjdefined in claim Zand further including the step offixing said developed images to said xerographic member.

4. The method as defined in claim Zand further includingthe steps oftransferring said developed images to a receiver material and fixingsaid developed images thereto. I

5. The method as defined in claim 1 wherein the insulating film andphotoconductive insulating layerof the xerographic member have anelectrical thickness relationship of from about 2:1 to about 1:2.

6. The method as defined in claim 1 wherein the insulating film and thephotoconductive insulating layer of the xerographic member have anelectrical thickness relationship of about 1:1.

7. The method as defined in claim 1 wherein the electrically insulatingfilm is transparent to said activating radiation.

8. The method as defined in claim 1 wherein the conductive substrate istransparent to said activating radiation.

9. The method as defined in claim 1 wherein said first and said secondelectrostatic charges applied to said xerographic member are of oppositepolarities from each other.

10. The method as defined in claim 9 wherein step (b) is carried out byselectively illuminating said charged xerographic member with a patternof activating radiation corresponding to an original positive image.

ll. The method as defined in claim 9 wherein step (b) is carried out byselectively illuminating said charged xerographic member with a patternof activating radiation corresponding to at least one original neg- 12ative image. I

12. The method as defined in claim 11 and further including the step ofdeveloping said electrostatic latent images with a development systemwhich develops absolute differences from a reference potential.

13. The method as defined in claim 1 wherein said first and saidsecondelectrostatic charges applied to said xerographic member are of the samepolarity.

14. The method as defined in claim 13 wherein step (b) is carried out byselectively illuminating said charged xerographic member with a patternof activating radiation corresponding to an original positive image.

IS. The method as defined in claim 14 and further including the step ofdeveloping said electrostatic latent images with a development systemwhich develops absolute'differences from a reference potential.

16. The method as defined in claim 13 wherein step (b) is carried out byselectively illuminating said charged xerographic member with a patternof radiation corresponding to at least one original negative image.

17. The method as defined in claim 16 and further including the step ofdeveloping said electrostatic latent images by depositing thereoverelectroscopic marking material having a charge of polarity opposite tothat of said first and second electrostatic charges.

2. The method as defined in claim 1 and further including the step ofdeveloping said electrostatic latent images by depositing electroscopicmarking particles thereon.
 3. The method as defined in claim 2 andfurther including the step of fixing said developed images to saidxerographic member.
 4. The method as defined in claim 2 and furtherincluding the steps of transferring said developed images to a receivermaterial and fixing said developed images thereto.
 5. The method asdefined in claim 1 wherein the insulating film and photoconductiveinsulating layer of the xerographic member have an electrical thicknessrelationship of from about 2:1 to about 1:2.
 6. The method as defined inclaim 1 wherein the insulating film and the photoconductive insulatinglayer of the xerographic member have an electrical thicknessrelationship of about 1:1.
 7. The method as defined in claim 1 whereInthe electrically insulating film is transparent to said activatingradiation.
 8. The method as defined in claim 1 wherein the conductivesubstrate is transparent to said activating radiation.
 9. The method asdefined in claim 1 wherein said first and said second electrostaticcharges applied to said xerographic member are of opposite polaritiesfrom each other.
 10. The method as defined in claim 9 wherein step (b)is carried out by selectively illuminating said charged xerographicmember with a pattern of activating radiation corresponding to anoriginal positive image.
 11. The method as defined in claim 9 whereinstep (b) is carried out by selectively illuminating said chargedxerographic member with a pattern of activating radiation correspondingto at least one original negative image.
 12. The method as defined inclaim 11 and further including the step of developing said electrostaticlatent images with a development system which develops absolutedifferences from a reference potential.
 13. The method as defined inclaim 1 wherein said first and said second electrostatic charges appliedto said xerographic member are of the same polarity.
 14. The method asdefined in claim 13 wherein step (b) is carried out by selectivelyilluminating said charged xerographic member with a pattern ofactivating radiation corresponding to an original positive image. 15.The method as defined in claim 14 and further including the step ofdeveloping said electrostatic latent images with a development systemwhich develops absolute differences from a reference potential.
 16. Themethod as defined in claim 13 wherein step (b) is carried out byselectively illuminating said charged xerographic member with a patternof radiation corresponding to at least one original negative image. 17.The method as defined in claim 16 and further including the step ofdeveloping said electrostatic latent images by depositing thereoverelectroscopic marking material having a charge of polarity opposite tothat of said first and second electrostatic charges.